The molding of thermoset polymers is a technologically important processing technique. In one version of this technique, a liquid monomer (e.g., an olefin) and a polymerization catalyst are mixed and poured, cast or injected into a mold. The polymerization proceeds (the article xe2x80x9ccuresxe2x80x9d) and on completion the molded part is removed from the mold for any post cure processing that may be required. The polymerization reaction mixture may optionally contain additional ingredients such as modifiers, fillers, reinforcements, and pigments.
The time during which the liquid monomer/catalyst mixture can be worked on after the monomer and catalyst are mixed is called the xe2x80x9cpot lifexe2x80x9d of the polymerization reaction mixture. In general, the ability to control reaction rates increases in importance in the molding of larger parts. To mold successfully, the reaction mixture must not cure so quickly that the liquid monomer/catalyst mixture polymerizes before the mixture can be introduced in to the mold or before the catalyst has had time to completely dissolve. However, for convenience and expedient cycle time, it is also important that the catalyst activate within a reasonable time after the mold is filled.
Reaction Injection Molding (xe2x80x9cRIMxe2x80x9d) has previously been used for the molding of polymer articles using a polymerization catalyst and olefin monomer (U.S. Pat. Nos. 4,400,340 and 4,943,621). In these previous processes, a metal (W or Mo) containing compound is dissolved in a first monomer stream. The monomer streams are then mixed and the metal containing compound and the alkyl aluminum compound react to form an active catalyst which then catalyzes the polymerization reaction. Because the reaction proceeds extremely quickly once the catalyst is formed, any attempt to modulate the polymerization time relies on delaying the formation of the active catalyst species. For example, the alkyl aluminum compound stream typically includes an inhibitor, usually a Lewis base, which suppresses the formation of the catalyst.
As molding processes tackle larger and more complicated polymeric components, there is an increasing need for more reliable systems which can extend pot life and/or control the rate of metathesis polymerization reactions.
The present invention addresses these needs by providing compositions for olefin metathesis reaction but whose reaction rate may be controlled. In general, the catalysts are vinylidene ruthenium and osmium complexes that are substantially inactive at a first temperature (preferably about room temperature) but become progressively more active at higher temperatures. This difference in reactivities allows the reaction mixture to be formed and manipulated at the first temperature until polymerization is desired. When appropriate, the reaction mixture is heated to a suitable second temperature (preferably greater than about 50xc2x0 C.) to activate the catalyst to initiate polymerization. In preferred embodiments, the heat activation occurs in bursts (as opposed to the continuous application of heat) so as to slow the reaction rate and to allow for a more complete incorporation of the monomers before crosslinking. Other than the requirement for heat activation, the inventive compositions may be used in a similar manner as known olefin metathesis catalysts, particularly ruthenium and osmium complex catalysts. Because the initiation and rate of polymerization may be controlled with temperature, the inventive methods are especially suitable for ring opening metathesis polymerization (xe2x80x9cROMPxe2x80x9d) reactions and for molding polymer articles that require extended pot-lives.